Within the field of semiconductor circuits, certain categories of circuitry require a reliable operation over a range of temperatures. One circuit that may be used to provide a constant reference source is a bandgap voltage reference.
Bandgap voltage reference circuits are well known in the art. Such circuits are designed to sum two voltages with opposite temperature slopes. One of the voltages is a Complementary-To-Absolute Temperature (CTAT) voltage typically provided by a base-emitter voltage (VBE) of a forward biased bipolar transistor. The other is a Proportional-To-Absolute Temperature (PTAT) voltage typically derived from the base-emitter voltage differences of two bipolar transistors operating at different collector current densities. When the PTAT voltage and the CTAT voltage are summed together the summed voltage is at a first order temperature insensitive.
In typical bandgap voltage references, the output voltage requires trimming or adjusting so as to achieve a constant output voltage reference over a range of temperatures. This is typically achieved by altering the PTAT voltage because it is more difficult to alter the CTAT voltage due to the exponential relationship between the current and the base-emitter voltage of a bipolar transistor. Typically, both the absolute voltage and the temperature slope of a bandgap voltage reference must be trimmed, with the assumption that the base-emitter voltage of the bipolar transistor has a precise value at absolute zero. The base emitter voltage at zero Kelvin is known as the bandgap voltage. Due to the process variations, both the output voltage and the temperature slope or temperature coefficient (TC) for a real bandgap voltage reference will have different values from device to device. This causes problems if a precise absolute voltage and minimum temperature coefficient are required. When the PTAT voltage is adjusted at room temperature so as to correct the temperature slope, the adjustment turns the slope around 0 Kelvin, which causes the absolute voltage to also change. Therefore, once the temperature slope has been corrected, the absolute voltage must also be corrected. This correction in absolute voltage may in turn alter the temperature slope. As a result, the trimming process typically requires the step of correction of the temperature slope followed by the step of correction of the absolute voltage to be repeated several times. This means that when a precise absolute voltage and minimum temperature coefficient is required, a lengthy iterative process of trimming slope and absolute voltage must be employed.
Another way to trim the reference voltage is to record a minimum of two reference voltage values at two different temperatures, in order to find the temperature slope, and then to adjust the PTAT voltage by a corresponding amount and shift the reference voltage (or the gain) with a temperature constant value. However temperature trimming of units in production quantities using this technique has the drawback of requiring multiple handling and tracking of the individual units during temperature test.
A number of techniques have been developed to provide for the compensation of the temperature effect. An example of such a technique is disclosed in U.S. Pat. No. 6,075,354 (the content of which is incorporated herein by way of reference). In this document, three currents DAC's are provided to interface with a bandgap voltage generator, a first provided to trim first order temperature slope variations of the output reference voltage, a second to compensate for temperature slope curvature and a third to provide scalar gain adjustment. In order to adjust ΔVBE for the slope correction, the technique is used of pushing an external correction current through the first or second diode of the main bandgap cell. A drawback of this scheme is that as the PTAT voltage changes, the reference voltage slope also changes, which affects the absolute value of the reference voltage.
U.S. Pat. No. 6,329,804 also describes a slope and level trim DAC for voltage references. In order to trim the reference voltage slope, a current switching DAC is used to inject a PTAT trimming current into one of the two diodes in the main bandgap cell. However, as in the case of U.S. Pat. No. 6,075,354, this patent also has the drawback that a change in ΔVBE changes both the slope and absolute value of the reference voltage.